I just bought these for my 93 xlt. I have a sony aftermarket 50 watt per channel head unit:
It might be that the clutch gap is a little too far and by hitting a bump in the road it catches and engages. Regardless, check the power on the clutch is on and shove the clutch in with a big screw driver if it is.
There are shim washers under the clutch.
This is just a guess, but it happens.
For $100 you can buy a vacuum pump and set of gauges on ebay. I doubt that you were holding a vacuum. Suck it down and hold it for an hour or so before you worry about R134a. If it holds for an hour...with the pump off and the gauges closed, maybe there are no leaks.